1. Field of the Invention
The present invention relates generally to an apparatus for polishing or planarizing semiconductor workpieces such as silicon wafers. More particularly, the present invention relates to a wafer carrier for planarizing or polishing wafers on a polishing pad.
2. Description of the Related Art
Silicon workpieces or wafers, which are typically flat and circular in shape, are used in manufacturing semiconductor devices. Wafers are initially sliced from a silicon ingot and, thereafter, undergo multiple masking, etching, and dielectric and conductor deposition processes to create microelectronic structures and circuitry. The surface of a wafer undergoing these processes typically are polished or planarized between processing steps to ensure proper flatness to facilitate the use of photo lithographic processes for building additional dielectric and metallization layers on the wafer surface.
Chemical Mechanical Planarization ("CMP") machines have been developed to polish or planarize silicon wafer surfaces to the flat condition desired for manufacture of integrated circuit components and the like. For examples of conventional CMP processes and machines, see U.S. Pat. No. 4,805,348, issued in February 1989 to Arai, et al.; U.S. Pat. No. 4,811,522, issued in March 1989 to Gill; U.S. Pat. No. 5,099,614, issued in March 1992 to Arai et al.; U.S. Pat. No. 5,329,732, issued in July 1994 to Karlsrud et al.; U.S. Pat. No. 5,476,414, issued in December 1995 to Masayoshi et al.; U.S. Pat. Nos. 5,498,196 and 5,498,199, both issued in March 1996 to Karlsrud et al.; and U.S. Pat. No. 5,558,568, issued in September 1996 to Talieh et al.
Typically, a CMP machine includes a wafer carrier configured to hold and to rotate a wafer during the polishing or the planarizing of the wafer. For example, with reference to FIG. 1, a conventional wafer carrier 100 includes an upper housing 101 and a pressure plate 104 mounted underneath a lower or secondary housing 106. A plurality of fasteners 108 fix pressure plate 104 to lower housing 106. A plurality of vacuum holes 110 hold the wafer to be planarized to the planar lower surface of pressure plate 104. Wafer carrier 100 then presses the wafer against a polishing pad (not shown) to polish or to planarize the wafer. More particularly, pressure plate 104 applies pressure to the wafer such that the wafer engages the polishing pad with a desired amount of pressure. The pressure plate and the polishing pad are also rotated, typically with differential velocities, to cause relative lateral motion between the polishing pad and the wafer to produce a more uniform thickness. Additionally, an abrasive slurry, such as a colloidal silica slurry, is often provided to enhance the polishing or planarizing process.
Conventional wafer carriers are typically rotated by a drive motor through a central drive shaft and a mechanical bearing assembly. For example, conventional wafer carrier 100 includes a bearing assembly 112 disposed between lower housing 106 and upper housing 101 and a drive shaft 114 connected to a drive motor (not shown). Bearing assembly 112 permits the movement of lower housing 106 and pressure plate 104 relative to upper housing 101 in order to maintain the surface of the wafer in parallel contact with the polishing pad even when the pad deviates from planarity. This motion is often referred to as "gimballing", and the "gimbal point" is defined as the intersection of the plane in which the pressure plate 104 gimbals and the vertical central axis of the carrier. The gimbal point of wafer carrier 100, for example, is at point 116. The location of the gimbal point above the lower or backing surface of the pressure plate, however, can result in excessive tipping of the wafer with respect to the polishing pad, thus causing uneven edge polishing and detracting from uniform pressure distributed across the wafer.
Another shortcoming of conventional wafer carriers which arc rotated by a central drive shaft is the lag in response time due to the inertia of the wafer carrier. For example, when a torque is initially applied to drive shaft 114 to begin to rotate wafer carrier 100, the mass of wafer carrier 100 results in a lag in response time of the wafer carrier 100. Accordingly, the outer diameter portions of the wafer carrier 100 may initially rotate slower than the inner diameter portions of the wafer carrier 100, thus contributing to uneven polishing or planarizing of the wafer. Additionally, the mass of wafer carrier 100 may result in undesired vibrations when the rotational speed of drive shaft 114 is increased or decreased, thus further contributing to uneven polishing or planarizing of the wafer.
An additional shortcoming of conventional wafer carriers is that the downward pressure applied to the drive shaft is not ideally distributed across the wafer. For example, in carrier 100, upper housing 101 is connected to outer ring 118 of bearing assembly 112 by fasteners 120, while inner ring 122 of bearing assembly 112 is connected to lower housing 106 by fasteners 124. Hence, the pressure distribution path is as follows: downward pressure applied from the drive shaft is transmitted into upper housing 101, transmitted through fasteners 120 and into outer bearing ring 118, transmitted through bearing assembly 112 to inner bearing ring 122, and transmitted through fasteners 124 to the narrow central body portion 126 of lower housing 106 and pressure plate 104. Consequently, the downward pressure is concentrated at the central portion of the wafer and effects excessive material removal in the inner diameter portions of the wafer, while bowing and inadequate removal occurs at the outside diameter portions of the wafer.